First step in the plan described in meta-bug #153488 would be to store grid lines indexes instead of track indexes in GridResolvedPosition. This will mean port the following patch from Blink: https://codereview.chromium.org/1451883002/
Created attachment 270114 [details] Patch
Comment on attachment 270114 [details] Patch Makes total sense
Comment on attachment 270114 [details] Patch Clearing flags on attachment: 270114 Committed r195808: <http://trac.webkit.org/changeset/195808>
All reviewed patches have been landed. Closing bug.